1. Field of the Invention
The present invention relates in general to manual transmissions for a motor vehicle, and more particularly to structures for mounting a reverse idler gear in the transmission.
2. Description of the Prior Art
For clarifying the task of the present invention, one conventional structure for mounting a reverse idler gear in a manual transmission will be described with reference to FIGS. 8 and 9 of the accompanying drawings.
In FIG. 8, there is shown a manual transmission to which the conventional reverse idler gear mounting structure is practically applied. The transmission is of a type having five forward speeds and one reverse.
The transmission comprises a transmission case 2 in which an input shaft 6 and a main shaft 10 are arranged in parallel with each other. The input shaft 6 is rotatably held by bearing units 4a and 4b installed in the transmission case 2, and the main shaft 10 is rotatably held by bearing units 8a and 8b installed in the case 2.
The input shaft 6 has a front end (or right end in FIG. 8) exposed to an interior of a clutch housing 2a which constitutes part of the transmission case 2. Within the clutch housing 2a, there is arranged a clutch (not shown) to which the front end of the input shaft 6 is connected. That is, when the clutch is engaged, a power from an engine (not shown) is transmitted to the input shaft 6 through the clutch. While, when the clutch is disengaged, the power transmission from the engine to the input shaft 6 is intercepted.
The input shaft 6 has first, second and fifth speed input gears 6a, 6b and 6e and a reverse input gear 6f which are integral with or connected to the input shaft 6 to rotate therewith. As shown, the reverse input gear 6f is formed on the left end portion of the input shaft 6. The input shaft 6 has further third and fourth speed input gears 6c and 6d which are rotatably disposed thereon. As shown, the six input gears 6a, 6b, 6c, 6d, 6e and 6f are aligned in order.
The main shaft 10 is formed at its front portion with an output gear 11 which is engaged with a final gear 18a of a differential gear unit 18. The main shaft 10 has first, second, third, fourth and fifth speed main gears 10a, 10b, 10c, 10d and 10e which are respectively engaged with the first, second, third, fourth and fifth input gears 6a, 6b, 6c, 6d and 64 of the input shaft 6. The first, second and fifth speed main gears 10a, 10b and 10e are rotatably disposed about the main shaft 10, while the third and fourth speed main gears 10c and 10d are connected to the main shaft 10 to rotate therewith. Rotatably disposed about the rear end of the main shaft 10 is a reverse main gear 10f. The reverse main gear 10f is engaged with the reverse input gear 6f of the input shaft 6 through a reverse idler gear 20 which is rotatably held in the transmission case 2.
Around the main shaft 10 between the first and second speed main gears 10a and 10b, there is arranged a 1-2 synchronizing mechanism 12, around the main shaft 10 between the fifth speed main gear 10e and the reverse main gear 10f, there is arranged a 5-R synchronizing mechanism 14, and around the input shaft 6 between the third and fourth speed input gears 6c and 6d, there is arranged a 3-4 synchronizing mechanism 16.
When, due to operation of selected one of shift forks (not shown), the synchronizing mechanism 12, 14 or 16 is actuated to achieve a fixed connection of the main gear 10a, 10b, 10e or 10f to the main shaft 10 or a fixed connection of the input gear 6c or 6d to the input shaft 6, a certain gear train is established in the transmission. With this, the engine power is transmitted to a drive shaft of the differential gear unit 18 while being changed in rotation speed and in a certain case in rotation direction.
With the above-mentioned arrangement, the manual transmission can be constructed compact in size and light in weight. This is because the reverse idler gear 20 has no need to move axially and the reverse mechanism (that is, the mechanism including the reverse input gear 6f, the reverse main gear 10f and the reverse idler gear 20) can be operated by actuating the 5-R synchronizing mechanism 14 which serves also as means for establishing the fifth gear speed.
In the reverse mechanism, the reverse idler gear 20 is rotatably disposed about an idler shaft 22 through a bearing unit 24. The idler shaft 22 extends in parallel with the input and main shafts 6 and 10.
As is shown in FIG. 9, the idler shaft 22 comprises a center shaft portion 22a about which the bearing unit 24 is arranged, a smaller diameter end portion 22b which coaxially extends from one end of the center shaft portion 22a, a larger diameter discal end portion 22c which coaxially extends from the other end of the center shaft portion 22a and a sectoral block portion 22f which extends from the discal end portion 22c. As shown, the sectoral block portion 22f has a cylindrical outer surface 22d which is flush with a peripheral cylindrical surface of the discal end portion 22c. The sectoral block portion 22f is formed at the cylindrical surface 22d with a threaded bolt hole 22e.
As is shown in FIG. 8, an end wall 2b of the transmission case 2 is formed at its inner surface with a smaller circular recess 2c into which the smaller diameter end portion 22b of the idler shaft 22 is tightly received. A side wall 2d of the transmission case 2 is formed at its inner surface with a concave part 2d.sub.1 with which the cylindrical outer surface 22d of the idler shaft 22 intimately engages. The side wall 2d is formed at the concave part 2d.sub.1 with a bolt opening 28 through which a threaded bolt 30 inserts into and engages with the threaded bolt hole 22e of the idler shaft 22. As is mentioned hereinabove, the reverse idler gear 20 is rotatably disposed on the idler shaft 22 through the bearing unit 24. A washer 26 is disposed on the smaller diameter end portion 22b to operatively hold both the reverse idler gear 20 and the bearing unit 24 on the idler shaft 22.
Although not well shown in FIG. 8, the side wall 2d of the transmission case 2 is further formed near the concave part 2d.sub.1 with a parts-inserting opening whose axial length is denoted by reference "L.sub.1 " in the drawing. The axial length "L.sub.1 " is greater than the axial length of the idler shaft 22.
For mounting the reverse idler gear 20 to a right position in the transmission case 2, the following steps are taken.
First, at open space, a semi-assembled unit is prepared. That is, the bearing unit 24, the washer 26 and the reverse idler gear 20 are attached to the idler shaft 22 to constitute the semi-assembled unit. Then, the semi-assembled unit is inserted into the transmission case 2 through the parts-inserting opening of the same. Then, the semi-assembled unit is manually set, so that the smaller diameter end portion 22b of the idler shaft 22 is put in the recess 2c of the end wall 2b and the cylindrical outer surface 22d of the sectoral block portion 22f of the idler shaft 22 is intimately mated with the concave part 2d.sub.1 of the side wall 2d. After registering the threaded bolt hole 22e of the sectoral block portion 22f with the bolt opening 28 of the side wall 2d, the threaded bolt 30 is inserted into the registered holes and engaged with the threaded bolt hole 22e. With this, the semi-assembled unit is tightly held in the transmission case 2 as is shown in FIG. 8. Upon assemblage of the unit, the reverse idler gear 20 is arranged between the reverse input gear 6f and the reverse main gear 10f while properly meshing with them.
However, due to its inherent construction, the above-mentioned reverse idler gear mounting structure has failed to satisfy the users because of the following reasons.
First, due to the complicated shape of the idler shaft 22 (see FIG. 9), production of it needs a highly precise and time-consumed machining work, which inevitably induces increased cost of the shaft 22, and thus, that of the entire of the reverse idler gear mounting structure.
Second, it is difficult or at least troublesome to insert the semi-assembled unit into the right position in the transmission case 2 through the parts-inserting opening. In fact, the parts insertion work by using only one parts-inserting opening has its limit.